An important element of the application of the teachings of color switch chemistry is the ability to switch high extinction coefficient chromophores. High extinction coefficient switchable dyes allow thinner colorant films that, in turn, require desirably lower pixel switching voltage for a given electric field in devices such as displays. Chromophores having high extinction coefficients are well known in the dye art and compose commercial dyes used throughout the world. It is, therefore, highly desirable to embody the teachings of intramolecular polarization and tautomerization technology into these commercial chromophores for switching purposes. Following a careful study of commercial chromophore chemistries, it was discovered that many desired switch solutions utilize or require variations of the polarization and tautomer concept not previously disclosed.
Azo chromophores compose roughly half of the commercial dyes used world-wide. In comparison to other commercial chromophores, the azo chromophore uniquely enables high extinction coefficient over a full design range of hue and chroma at relatively low dye cost. For this reason, a color switch design around the azo chromophore is highly desirable. The azo group is normally a bridge group between aromatic rings and is preferably the switching group to break or disrupt π electron delocalization between the aromatic rings. This color switching is most easily accomplished by azo tautomerization to a secondary amine. An azo group attached to an aromatic ring has a relatively high electronegativity (0.19) and is, therefore, an electron acceptor that may change polarization and affect the tautomer state when coupled to an electric field. The azo group in a color switch, therefore, may be used as both an acceptor and tautomer, and is located central to the dye molecule, between conjugation units.
Certain dyes, for example those based on the aminonaphthylimide chromophore, have a neutral to charge polarized state tautomer that occurs entirely by intra-molecular charge separation within the tautomer. In some cases, the energy difference between the two tautomer states is sufficiently small that the tautomer reversibly and continuously switches between tautomer states at room temperature. In the case of the aminonaphthylimide chromophore, the charged state forms a π conjugation link between two aromatic groups while the uncharged state breaks the conjugation link. In this instance, both the acceptor and donor comprise the tautomer group.
Many dye structures are large in nature, include multiple chromophore units (e.g., disazo, trisazo) and auxochromes. Such large structures, or even small structures, can require transformation energies greater than can be coupled into the molecule through a single acceptor-donor and an electric field of desired intensity and below the dielectric strength of the material set. Further, dye chromophores often exhibit dichroism, wherein the extinction coefficient of the dye varies depending on the orientation of the molecule with the observed optical axis. Typically, the extinction coefficient diminishes as the length-wise axis of the colorant molecule aligns with the optical axis. Dye structures are known wherein the acceptor-donor induced dipole of the colorant molecule aligns with the length-wise axis of the molecule, which in turn is aligned with both the electric field and optical axis. In some instances, this orientation produces maximal extinction coefficient loss due to dichroism. For such cases, it is highly desirable to align the acceptor-donor axis orthogonal to the length-wise axis of the colorant molecule. Such an orientation, however, effectively prohibits a single electron acceptor-donor pair from effecting conjugation along the entire length of the colorant molecule. It is thereby desirable in such cases to include multiple acceptor-donor pairs within a given colorant molecule to improve field energy coupling, dichroism or both. Molecular designs have been taught wherein a single electron-accepting group and a single donor group are structured with at least one tautomerizable atomic group and at least one conjugating fragment. Still further, the use of multiple acceptor-donor/tautomer groups along a molecule is desirable for polymeric switchable colorants and colorants having more than two color states. Polymeric color switches promise the ability to switch between very highly conjugated color states, such as black, and very narrowly conjugated color states, such as found in transparent molecules. Polymeric color switches also offer colorant films in which the switchable colorant and holding polymer matrix are one and the same and thereby potentially improve color density per film thickness. Colorants having more than two switchable color states promise, for example, the ability to separately display each of a set of trichromatic colors in a single pixel.
Certain tautomers undergo a change in conformation from one tautomer state to another. Such conformation change can create steric hindrance or proton transfer distances that interfere with reversible switching. It is therefore desirable to select tautomer groups that maintain conformation and small proton transfer distances between tautomer states. Tautomers contained within a ring or an open ring can provide these properties. For example, a cyclic amine-imine tautomer maintains conformation and an approximately one angstrom (Å) proton transfer distance when tautomerizing from the amine to imine form.